Brominated polystyrenes have been found useful as flame retardants for thermoplastic polymers such as engineering thermoplastics. Generally, brominated polystyrenes are produced by a reaction between polystyrene and a brominating agent (e.g., bromine or bromine chloride) in the presence of a solvent (e.g., dichloroethane) and a Lewis acid catalyst. Heretofore the art has proffered many processes which are claimed to produce a superior brominated polystyrene. See U.S. Pat. Nos. 4,200,703; 4,352,909; 4,975,496 and 5,532,322.
Despite these efforts, previously-known brominated polystyrene flame retardants remain deficient in certain properties which translate into deficient performance of thermoplastic polymer blends in which they are used when the blends are subjected to polymer processing conditions.
For example, prior art brominated polystyrene polymers that have been evaluated for thermal stability have exhibited a 1% weight loss at temperatures less than 336.degree. C. when submitted to Thermogravimetric Analysis (TGA) and, indeed, most have exhibited a 1% weight loss at temperatures around 300.degree. C. Such low thermal stabilities are undesirable, especially under the high temperatures to which flame retarded engineering thermoplastics formulated with such brominated polystyrene polymers are exposed during processing.
Corrosion of metal processing equipment such as melt blenders, extruders, and molding machines, attributable to the release of hydrogen halide under thermal processing conditions is another deficiency of flame retarded thermoplastic polymer blends made using prior brominated polystyrene flame retardants. In the presence of moisture, hydrogen chloride and hydrogen bromide released from the brominated polystyrene in the blend during exposure to the elevated polymer processing temperatures can result in acid formation and consequent metal corrosion.
The bromine content of a brominated polystyrene is the sum of (1) the bromine which is substituted onto the aromatic portions of the polymer, (2) the bromine which is substituted onto the aliphatic portion of the polymer, e.g., the polymer backbone or aliphatic groups present due to alkylation of the aromatic portion of the polymer, and (3) any ionic bromine present, e.g., sodium bromide. The alkylation of aromatic rings in the brominated polystyrene is catalyzed by the Lewis acid catalyst used in producing the brominated styrenic polymer, and the reaction solvent (usually a 1-3 carbon atom dihaloalkane) serves as the alkylating agent. The bromine for (1) is referred to herein as aromatic bromide, while the bromine for (2) is referred to as aliphatic bromide. Even though ionic bromine can contribute to the bromine content, its contribution to the total bromine content is small.
The chlorine content of brominated polystyrenes is credited to chlorine which, like the bromine, is chiefly part of the polymer structure as an aromatic and/or an alkyl chloride. The use of bromine chloride as the brominating agent is the largest contributor to the chlorine content. However, chlorinated solvents and/or chlorine-containing catalysts used in the production of the brominated polystyrene may also contribute to the chlorine content of the brominated polystyrene.
The alkyl halide content of the brominated polystyrene is not desirable as alkyl halide is not as thermally stable as aromatic halide and, thus, alkyl halide can be easily converted to hydrogen halide, e.g., HBr or HCl, under normal end-use processing conditions. Alkyl bromide and chloride are generally referred to by the art and quantified, respectively, as hydrolyzable bromide and hydrolyzable chloride since such halides are easily hydrolyzed as compared to aromatic halides.
There are two different ways of evaluating brominated styrenic polymers for their tendencies to release hydrogen halide under thermal processing conditions. One method measures the hydrolyzable bromine content of the brominated polystyrene polymer. The other is a method described in U.S. Pat. No. 5,726,252 and referred to therein as the Thermal Stability Test Procedure. In essence, these methods indicate the content of halogen atoms in the brominated polystyrene that is not bonded directly to the aromatic rings and, thus, is more readily released from the polymer when at elevated temperature. Both such methods are described in greater detail hereinafter.
Apart from whether the halide is present as an aromatic or alkyl halide, it is also desirable to minimize the total chlorine content of the brominated polystyrene as chlorine is not as efficacious or as stable a flame retardant constituent as is bromine.
Additionally, it has been demonstrated that prior art processes for the manufacture of brominated polystyrene give rise to significant cleavage of the polymer chain. This cleavage results in the produced brominated polystyrene having an M.sub.w, as measured by Gel Permeation Chromatography, which is significantly lower than the calculated theoretical M.sub.w, of the brominated polystyrene. The calculation is based upon the bromine content (wt %) of the brominated polystyrene product and the M.sub.w, of the polystyrene reactant at reaction initiation. It is advantageous if the theoretical M.sub.w and the actual M.sub.w of the produced brominated polystyrene are close to each other, given the .+-. margins of error for GPC, since such closeness evidences a paucity of polymer cleavage. The degree of cleavage should be minimized since cleavage results in an increase of alkyl end groups in the brominated polystyrene, which alkyl end groups provide loci for the facile formation of the undesirable hydrolyzable halides discussed above.
The presence of conventional relatively high amounts of hydrolyzable halide in brominated polystyrenic flame retardants translates into formation and release of excessive amounts of corrosive hydrogen halide during thermal processing of thermoplastic polymers flame retarded with such materials. As an inevitable consequence of such hydrogen halide release in the presence of moisture is excessive corrosion of the apparatus used in such processing.
Thus, a need exists for thermoplastics, especially engineering thermoplastics, and particularly nylon (a.k.a. polyamide) thermoplastics, flame retarded by a brominated polystyrene, having enhanced thermal stability and reduced metal corrosiveness.